Thermal management towards ultra-bright and stable perovskite nanocrystal-based pure red light-emitting diodes

Despite the promising candidacy of perovskite nanocrystals for light-emitting diodes, their pure red electroluminescence is hindered by low saturated luminance, severe external quantum efficiency roll-off, and inferior operational stability. Here, we report ultra-bright and stable pure red light-emitting diodes by manipulating Joule heat generation in the nanocrystal emissive layer and thermal management within the device. Diphenylphosphoryl azide-mediated regulation of the nanocrystal surface synergistically enhances the optical properties and carrier transport of the emissive layer, enabling reduced Joule heat generation and thus lowering the working temperature. These merits inhibit ion migration of the CsPb(Br/I)3 nanocrystal film, promising excellent spectra stability. Combined with the highly thermal-conductive sapphire substrates and implementation of pulse-driving mode, the pure red light-emitting diodes exhibit an ultra-bright luminance of 390,000 cd m−2, a peak external quantum efficiency of 25%, suppressed efficiency roll-off, an operational half-life of 20 hours, and superior spectral stability within 15 A cm−2.

The PLQYs of the NCs were measured by a home-designed system 16,17 .A system consisting of a xenon flash lamp, a QEPro spectrometer, and a home-designed integrating sphere was used to measure the absolute PLQYs.The wavelength and power density of excitation light were 450 nm and ~0.3 mW cm -2 , respectively.The PL intensity was integrated within the wavelength of 550 to 750 nm.
For excitation density-dependent measurements 18 , the excitation light source was changed to a 405 nm continuous laser, which was modulated by a lock-in amplifier (SR830, Standard Research System).The PL intensity was recorded by a photodetector (Thorlabs PDA100A) that connected with the lock-in amplifier.A long-pass filter of 420 nm was placed right before the photodetector to filter the excitation light.50% of the excitation light was irradiated on the sample through a beam splitter, and the rest was reflected on another photodetector to monitor the intensity of the excitation light.An attenuator was put before the excitation light source to control the power density on the samples ranging from 0.1-100 mW cm -2 .The PLQY, i.e., PLQY at a given power density ( mW cm -2 ) of 405 nm excitation, can be determined by where, , 0.3,0.3are the PL intensity at an excitation of  mW cm -2 , the PL intensity at an excitation of 0.3 mW cm -2 , and the absolute PLQY determined by integrating sphere system, respectively.
For the calculation of the nonradiative recombination rates, PL decays were fitted by the biexponential function: where 0, 1, and 2 are constants,  is time and 1, 2 are the decay times.The average PL lifetime () was calculated as: Where: Photoluminescence quantum yield in low-dimensional perovskite under low excitation power density can be described by the following formula: The carrier decay time and carrier recombination rate (kr, knr) have the following relationship: Thus, the radiative recombination rate: and the nonradiative recombination rate 19 :

Ion migration activation energy measurement
The decay in the temperature-dependent temporal response can reflect the kinetics of ionic movement.The temperature-dependent ionic conductivity  was addressed as 20 : Where Ea is the activation energy for ion transport, kB is the Boltzmann constant.The decay rate (k = τ -1 ) of the current decay represents the ionic transport dynamics and is proportional to the ionic conductivity.Therefore, the temperature-dependent decay rates can be used to obtain the thermal active energy for ion migration.
The ion migration activation energy measurement was carried out on the device with a structure of glass/NCs/Au.The NC solution (~15 mg mL -1 in octane) was spun onto the glass at 4000 rpm for 45 s.Then the Au interdigital electrode (80 nm) was deposited onto the NC film by thermal evaporation under a high vacuum (< 5 × 10 -4 Pa).The samples were placed on a liquid nitrogen thermostat, and Keithley 2635B was used to detect the current signal.The current decay curves were obtained at 12 V and fitted by a double exponential function.The τ1 is independent of temperature, which relates to the equipment response.The τ2 represents the time constant of ion migration and is used for calculating the activation energy.The ion migration activation Ea was calculated as the slope of ln(kT)-1/T using the relation where k can be obtained by k = τ -1 using the time constant τ2 at different temperatures from 255 K to 305 K.

DFT calculations
We utilized the first-principles tool, the Vienna Ab initio Simulation Package (VASP) 21,22 , for all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) employing the Perdew-Burke-Ernzerhof (PBE) 23 formulation.Projected augmented wave (PAW) potentials 24,25 were chosen to describe the ionic cores, incorporating valence electrons using a plane wave basis set with a kinetic energy cutoff of 450 eV.Partial occupancies of the Kohn−Sham orbitals were permitted using the Gaussian smearing method with a width of 0.05 eV.For geometry and lattice size optimization, Brillouin zone integration was conducted with 1×1×1 Γ-centered k-point sampling 26 .Self-consistent calculations applied a convergence energy threshold of 10 -5 eV.
Equilibrium geometries and lattice constants were optimized with a maximum stress on each atom within 0.02 eV Å -1 .The 35 Å vacuum layer was typically added to the surface to eliminate artificial interactions between periodic images, with the bottom 3 atom layers fixed.Weak interactions were described using the DFT+D3 method employing empirical correction in Grimme's scheme 27,28 .The spin polarization method was adopted to characterize the magnetic system.Furthermore, the crystal orbital Hamilton population (COHP) was calculated using the Lobster too [29][30][31][32]

Supplementary Fig. 1 |
Reaction processes between DPPA and precursor solvent.a, Schematic diagram of the reaction between DPPA and OA, followed by the subsequent reaction between the resulting acyl azide intermediate and OAm.b, c, d, Verification of the reaction process by FTIR spectra.(b) FTIR spectra of OA, DPPA, resultant of DPPA and OA, and DPP, respectively.The orange range indicates the stretching vibration of the aromatic ring, the blue range represents the stretching vibration of P=O bonds, and the green region indicates the stretching vibration of P-O bonds.(c) The enlarged FTIR spectra.The left depicts the enlarged infrared spectrum of the -N3 stretching vibration, demonstrating a high-frequency shift after adding OA compared to pure DPPA.This shift is attributed to changes in adjacent functional groups, leading to an inductive effect.The right shows the enlarged infrared spectrum of the benzene ring, which also exhibits frequency variations.Furthermore, the peak positions of the benzene ring in DPPA+OA resultant align with those of pure DPP, indicating the formation of DPP from the reactants DPPA and OA.These results validate the reaction in Step Ⅰ.(d) FTIR spectra of OAm, DPPA+OA, and DPPA+OA+OAm.Further addition of OAm results in a distinct peak at 1,554 cm -1 , corresponding to the vibration of the amide bond in the secondary amide, validating the process outlined in Step Ⅱ.

Table 4 | Summary of reported red PeLEDs with superior performance.
a The T50 is summarized at the initial luminance of 100 cd m -2 .

Table 5 | The fitting data from the current decay plots.
The pump beam was blocked using a polarization plate.The instrument response function of the system is about 30 fs by measuring the probe light intensity change of a blank sample.